1. Field of the Invention
The present invention generally relates to CCD (charge coupled device) solid state image sensing devices and method of forming a CCD solid state image sensing device and, more particularly, is directed to a structure of a photo-sensitive section formed by a PN junction of a semiconductor.
2. Description of the Prior Art
In general, a photo-sensitive section of a CCD solid state image sensing device is formed of a photodiode by a PN junction of a semiconductor.
FIG. 1 shows an example of a photo-sensitive section according to the prior art in which an N-type impurity diffusion region 22 is formed on the surface of a P-type silicon substrate and a photodiode PD is formed by a PN junction J of the silicon substrate 21 and the N-type impurity diffusion region 22.
FIG. 2 shows an example of a photo-sensitive section according to the prior art in which the N-type impurity diffusion region 22 is formed on the surface of a P-type well region 24 formed on an N-type silicon substrate 23 and the photodiode PD is formed by the PN junction J of the P-type well region 24 and the N-type impurity diffusion region 22. The above-mentioned photodiode PD constructs a photo-sensitive section 25 of a CCD solid state image sensing device.
Then, the above N-type impurity diffusion region 22 is formed by the ion implantation technique of phosphor (P) in the prior art.
However, in the conventional CDD solid state image sensing device, since the N-type diffusion region 22 constructing the photo-sensitive section 25 is formed by the ion implantation technique of phosphor (P), there is then the disadvantage such that a lattice distortion and stress tends to occur on the PN junction plane J formed by the contact of the N-type diffusion region 22 with the P-type silicon substrate 21 or the P-type well region 24.
This is a phenomenon which tends to occur because an atomic radius of the crystal of phosphor (P) is small as compared with that of silicon Si. This phenomenon exerts an influence on an electric characteristic of the CCD solid state image sensing device manufactured. More specifically, due to the lattice distortion and the stress, a leakage current of the reverse direction is increased on the plane of the PN junction J and a dark current is locally increased (i.e., a so-called fluctuation of dark current). There is then the problem such that a white noise, which is one of the defects encountered with an imaging screen, tends to occur.
Further, in the case of phosphor (P), its diffusion coefficient is large so that the phosphor (P) is diffused over a wide region by the activation treatment (heat treatment) after the ion implantation process of phosphor (P). As a consequence, the N-type impurity diffusion region 22 cannot be reduced in size, which brings a disadvantage in making the CCD solid state image sensing device compact in size.